GERHARD LUDWIG CLOSS 1928–1992 · three other fields of chemistry, one area for each decade of...

N A T I O N A L A C A D E M Y O F S C I E N C E S

G E R H A R D L U D W I G C L O S S1 9 2 8 – 1 9 9 2

A Biographical Memoir by

H E I N Z D . R O T H

Biographical Memoirs, VOLUME 84

PUBLISHED 2003 BY

THE NATIONAL ACADEMIES PRESS

WASHINGTON, D.C.

Any opinions expressed in this memoir are those of the authorand do not necessarily reflect the views of the

National Academy of Sciences.

3

GERHARD LUDWIG CLOSS

May 1, 1928–May 24, 1992

B Y H E I N Z D . R O T H

WHEN GERHARD LUDWIG CLOSS succumbed to a massive heartattack on May 24, 1992, the work of one of the great

organic chemists came to an untimely end. Professor Clossmade significant contributions in four areas. He was anearly leader in the field of carbene chemistry; he elabo-rated various significant aspects of the photosynthetic pig-ments; he pioneered important applications of magneticresonance to characterize reaction intermediates; and heelucidated intricate facets of electron transfer chemistry.This biographical memoir provides a welcome opportunityto pay tribute to one of the outstanding chemists of thepost-World War II era—and to a friend.

Gerhard Closs was born on May 1, 1928, in Wuppertal-Elberfeld, Germany, a small, bustling city known for its sus-pended tram (Schwebebahn) and for the pharmaceuticalbranch of Bayer, one of Germany’s major chemical manu-facturers. Before he could complete his high-school educa-tion he was pressed into military service as a 16-year-old in1944. He barely survived the ordeal of war: He was seriouslywounded on the eastern front. After the war he completedhigh school and enrolled in Universität Tübingen. Havingreceived a Ph.D. degree in 1955 for work with Georg Wittig,he joined R. B. Woodward’s group at Harvard for two years.

4 B I O G R A P H I C A L M E M O I R S

In 1957 he accepted a position as assistant professor at theUniversity of Chicago as a natural products chemist.

Gerhard’s early independent studies were assisted byLieselotte E. Closs (née Pohmer), his wife and most pro-ductive coworker; their collaboration produced 15 publica-tions between 1959 and 1969. Lieselotte also received aPh.D. degree with Wittig for her classic work on the tran-sient existence of dehydrobenzene.1,2 She did postdoctoralwork at MIT. Lieselotte and Gerhard were married on Au-gust 17, 1956, in Cambridge, Massachusetts.

Employment opportunities for women scientists were verylimited in the 1950s and 1960s. University regulations againstnepotism prohibited wives from holding paid positions inthe same department as their husbands; therefore, Lieselottecould only work as an unpaid volunteer. The availability ofa skilled coworker proved especially fortuitous when Gerhardentered the chemically induced dynamic nuclear polariza-tion field (see below). Lieselotte re-entered the lab, carriedout a few simple but elegant experiments to probe key as-pects, and soon had results sufficient for two “Communica-tions to the Editor.”

Gerhard Closs was granted tenure in 1961 and was pro-moted to full professor just two years later. Almost 20 yearslater he accepted the position of section head in the Chem-istry Division at Argonne National Laboratory, while remain-ing on the Chicago faculty. Although he kept this positionfor only three years, it significantly influenced the direc-tion of his research in the final decade of his life.

The work of Gerhard Closs has been recognized at theUniversity of Chicago and in the scientific community atlarge. He was appointed the Michelson Distinguished Ser-vice Professor, and his colleagues honored him along withN. C. Yang with a symposium on the occasion of their sixti-

5G E R H A R D L U D W I G C L O S S

eth birthdays. He was awarded the Jean Servas Stas Medalby the Belgian Chemical Society in 1971, the James FlackNorris and A. C. Cope awards by the American ChemicalSociety in 1974 and 1991, respectively, and the Photochem-istry Prize by the Inter-American Photochemical Associa-tion in 1992. He was elected a member of the NationalAcademy of Sciences in 1974 and the American Academy ofArts and Sciences the following year. The Inter-AmericanPhotochemical Association honors his memory with the G.L. Closs Memorial Award, which allows a student to presenta research paper at one of its meetings.

In 1981 he also was honored as chairman of the GordonResearch Conference on free radical reactions and in 1990the Gordon Research Conference on radical ions, and bymany distinguished lectureships in the United States, Canada,Japan, and Europe. Among these were the Bayer Lecture-ship at Universität Köln, Germany (where the author firstmet him), a visiting professorship at Yale (where the ac-quaintance was renewed), regular visits to Bell Laborato-ries, and the Merck Distinguished Lectureship at RutgersUniversity (where the author was privileged to be his host).

Closs was a featured speaker at many national and inter-national congresses and symposia, and his participation atmeetings was a highly important contribution to science.He brought to these meetings a keen analytical mind andthe command of an unequalled breadth of chemical topics:from subtle details of organic synthesis, to a deep under-standing of mechanistic details, to the intricacies of chemi-cal physics, and a keen chemical intuition. This combina-tion allowed him to probe proposed theories or mechanismsas they were being presented. Few of his peers made morepertinent comments than Gerhard did, or in a more imper-tinent fashion when he felt it necessary. Even accomplished


and experienced lecturers must have felt a tinge of appre-hension when he raised his hand and, on being recognized,uttered his familiar, “I would like to take issue with . . .”

Gerhard Closs relaxed by sailing, and sometimes racing,his sailboat on Lake Michigan for hours, days, or weeks. Herelished his fine collection of graphic art, he was stimu-lated by theater performances, including modern and avant-garde plays, and he enjoyed classical music. No matter howhard the author tried, however, Gerhard could not be per-suaded to attend an opera performance. (He had sworn offopera as a teen in the early 1940s, following a performanceof Da Ponte and Mozart’s Cosi Fan Tutti in Wuppertal.)

During his 35 years at the University of Chicago, Gerharddeveloped a deep appreciation, even love, for his adoptedcountry. He only bought American cars and “took issue”with many Americans and foreigners alike who dared tocriticize the United States in his presence. One afternoon,while attending a Gordon Conference in New Hampshire,Gerhard was interviewed by a local reporter. Asked what hethought of consumers who bought foreign-made articles,he quickly voiced his disapproval and then added, havingspotted the reporter’s Japanese-made camera, “and that ap-plies also to you.”

With the death of Gerhard Closs the chemical scienceslost a most formidable champion, a practitioner of the highestintellectual standards, a keen mind, and a skilled experi-menter who was always probing accepted theories and wasnever afraid to break new ground. The scientific commu-nity has lost a teacher, mentor, collaborator, and kin spirit,and a few who were privileged have lost a friend.

The earliest publications of Gerhard Closs stem fromhis thesis work with Wittig and describe ylid rearrangementswith ring enlargement or contraction,3 and from hispostdoctoral training with R. B. Woodward (total synthesis


of chlorophyll).4 His first independent publications, for ex-ample, a paper on the active constituents of Panaeolusvenenosus,5 reflect his being hired as a natural productschemist. As part of the venenosus project the physiologicaleffects of the mushroom were to be tested, and the youngassistant professor volunteered for the study. The highlyamusing conversation ensuing between Gerhard Closs andhis physician and collaborator is part of the Mycologia pub-lication.5 It was often cited at Closs group festivities andnever failed to amuse; coworkers fortunate to have obtaineda reprint of this paper count it among their prized posses-sions.

Gerhard Closs never lost interest in natural productsand photosynthetic pigments. Seventeen of his 132 lifetimepublications dealt with the chlorophylls; he contributed sig-nificantly to such important topics as linked chlorophylldimers, photosynthetic reaction centers, and porphyrin metalcomplexes. Still, his most significant contributions came inthree other fields of chemistry, one area for each decade ofhis professional career.

Gerhard Closs’s first major contributions came in theemerging field of carbene chemistry; interestingly, a distantpredecessor at the University of Chicago, John U. Nef, wasan early champion of divalent carbon chemistry. Alas, Nef’sinterpretations of his results are at variance with the ac-cepted definitions and the prevailing understanding in thefield since the mid-twentieth century so that his work nolonger qualifies as carbene chemistry.6

The actual roots of the carbene field lie in the base-catalyzed hydrolysis of trichloromethane by Geuther in 1862.7

Hine repeated this experiment in 1949 and recognized thereaction as an α-elimination, the consecutive removal of H+

and Cl– from the same carbon, generating dichlorocarbene.8

In 1954 Doering and Hoffman trapped the postulated spe-


cies by addition to cyclohexene, demonstrating its intermo-lecular reactivity.9

The development of divalent carbon chemistry involvedvarious facets: substituted carbenes; the notion of spin mul-tiplicity; chemical studies probing carbene reactivity andthe stereochemistry of their reactions; and the applicationof new physical methods (e.g., electron spin resonance, elec-tron nuclear double resonance, chemically induced dynamicnuclear polarization, and optical spectroscopy). Gerhard Clossplayed a significant role in introducing these new techniquesto the study of carbenes.

Closs generated chlorocarbene from methylene chloride;addition of the new carbene to alkenes, benzene, or phe-nol gave rise to chlorocyclopropanes,10 tropylium chloride,11

or tropone,12 respectively. Five-membered heterocycles (e.g.,pyrrole and indole) reacted with chlorocarbene by ring ex-pansion.13 It is tempting to see in these ring enlargementsechoes of his doctoral thesis.

:CHClCl

NH

:CHCl

N

+

–

CHCl3 + OH– CCl3– + HOH (eq. 1)

CCl3– :CCl2 + Cl– (eq. 2)

:CCl2

ClCl

(eq. 3)


Additional carbenes arose by reaction of alkyl and benzalhalides with organolithium compounds; here the term“carbenoid” was introduced to denote carbenes that appearedto be complexed (i. e., associated) with lithium halide.14

The base-induced α-elimination of chloroalkenes formedcyclopropenes by intramolecular addition of alkenyl-carbenes.15

By the time of his promotion to associate professor hebegan to ask further-reaching questions; he decided to char-acterize carbenes more thoroughly and, if possible, observethem directly. Spectroscopic techniques available at this timeincluded optical spectroscopy and electron spin resonance.Optical spectroscopy had received a recent boost by theadvent of flash photolysis in 1949-50.16 Herzberg observedthe emission spectra of the parent methylene, CH2, and itsisotopomers CHD and CD2, in 1961.17

Electron spin resonance (ESR) spectroscopy was a laterdevelopment,18 but by 1953 organic free radicals or radicalions had been studied. Gerhard Closs was fortunate to haveClyde Hutchison, an expert ESR spectroscopist, as a col-league. They generated diphenylmethylene at cryogenic tem-peratures in benzophenone crystals and observed the firstESR spectrum of a ground state triplet carbene in a singlecrystal.19 About two weeks before the Chicago group, EdelWasserman and coworkers at Bell Laboratories generateddiphenylmethylene in a glassy matrix.20 The single crystalapproach of the Chicago collaborators lent itself to a moredetailed analysis and interpretation. Ultimately, electronnuclear double resonance (ENDOR) revealed the detailedstructure of this intermediate (see Figure 1).21 Among Closs’sadditional ESR studies cyclopentanediyl and trimethylcyclo-propenyl deserve special mention.


Following the work on the ESR spectroscopy of tripletstates Gerhard Closs studied triplet carbenes by optical spec-troscopy,22 including the pioneering time-resolved laser spec-troscopy study of diphenylmethylene.25 Two other groupsprobed optical spectroscopy of diphenylmethylene indepen-dently,23,24 and only one study had dealt with the applica-tion of time-resolved laser spectroscopy (TRLS) to carbenechemistry26 when Closs and Rabinow’s study of diphenyl-methylene addition to alkenes25 opened the field to studiesin other laboratories. Although the limited (µs) time reso-lution of these early studies appears almost primitive com-pared to today’s sophisticated TRLS experiments, the avail-able time resolution was exactly right for the somewhat“sluggish” diphenyl-methylene.

Related studies involved cyclopropenes and bicyclo-butanes, newly accessible with his new carbenes, notablythe isomerization of 2,4-dimethylbicyclo[1.1.0]butane tobutadiene. The conservation of orbital symmetry, a conceptdeveloped by Woodward and Hoffmann in the early 1960s,27

FIGURE 1 Structure of diphenylmethylene as derived from ENDOR experi-ments.21


predicts that this reaction will proceed as a concerted[σ2s+σ2a] process with predictable stereochemistry for themigrating carbon centers. Closs and Pfeffer probed the re-arrangement of two 2,4-dimethylbicyclobutanes to twohexadienes and elucidated the steric course of this reac-tion.28

By the mid-1960s Gerhard Closs, an acknowledged ex-pert in carbene chemistry, made his final major contribu-tion to this field, the first application of the chemicallyinduced dynamic nuclear polarization method. In 1967 en-hanced nuclear magnetic resonance (NMR) emission wasobserved in some chemical reactions,29,30 yet another facetin the rich palette of NMR applications. Because some re-actions giving rise to NMR emission were known free-radi-cal reactions, these effects were explained as electron-nuclearcross relaxation, hence the designation “chemically induceddynamic nuclear polarization” (CIDNP) for the new phe-nomenon. However, this mechanism was soon found want-ing, as an increasing number of effects were incompatiblewith the cross relaxation mechanism.

Gerhard Closs immediately recognized the value of thistechnique. With his thorough understanding of organic re-action mechanisms and his expertise in the physical prin-ciples underlying magnetic resonance, he was in a unique


position to elucidate the physical and chemical principlesunderlying CIDNP. He entered the field with all his vigor.He persuaded Lieselotte to return to the bench for a fewwell-designed experiments.31,32 In 1969 four back-to-backcommunications appeared in the Journal of the AmericanChemical Society, followed quickly by six more, for a totalof ten communications in only 20 months. After two CIDNPstudies in photoreactions of diphenyldiazomethane31 andbenzophenone32 Closs began to probe the actual origin ofthe spin polarization effects.

Recognizing that all CIDNP effects required the involve-ment of radical pairs,33 he developed a theory that couldexplain the observed polarization and designed elegant ex-periments to probe key features of the theory. Because thepolarization changed with the spin multiplicity (µ) of theprecursor from which the pair was generated, he suggestedCIDNP “as a tool for determination of spin multiplicities ofradical pair precursors.”34

He also recognized that the polarization pattern of CIDNPeffects depends critically on the relative g factors (∆g) ofthe paired radicals and illustrated the effect of systematicalchanges in the g factor difference (∆g) between the inter-acting radicals on the CIDNP spectra (see Figure 2).35

H

HC6H4-Y

C6H4-XHO

Y-C6H4H

O

X-H4C6

H C6H4-Y

C6H4-YH

• •

• • hν

In this context I will share an anecdote about the 1970American Chemical Society meeting in Houston, Texas, where


FIGURE 2 CIDNP spectra (benzylic protons) of coupling products gener-ated during the photoreactions of benzaldehyde (X = H) and derivatives(X = Cl, Br) with diarylmethanes (Y = H, Cl, Br).33


Gerhard first disclosed major features of the proposed theory.The ACS Organic Division sponsored a symposium aboutthe new phenomenon; most of the early contributors werepresent, except the Japanese and Russians. Gerhard wasscheduled to give the opening lecture of the afternoon ses-sion. After brief introductory remarks he asked for the firstslide, studied it briefly, asked quickly for the second andthird slides, then turning to the session chairman, said,“Mr. Chairman, I know this is highly irregular; I thoughtthis could only happen to Michael Dewar [Professor MichaelDewar, for four years, 1959-63, Closs’s colleague at the Uni-versity of Chicago; University of Texas, Austin 1963–]. Itook the wrong set of slides.” The next speaker was asked topresent his talk out of turn, and Gerhard retrieved the cor-rect set of slides from his hotel, which fortunately was justacross the street. When he returned, he showed a series ofastonishing slides that were well worth the (brief) wait (aswell as the violation of the ACS guidelines for the presenta-tion of papers).

Coincidental with the Chicago group, L. Oosterhoff andR. Kaptein at Universiteit Leiden (Netherlands) worked onthe mechanism of CIDNP and demonstrated additional ele-ments of the theory. They noted that in-cage products andcage-escape (free-radical) products showed polarization ofopposite sign,36 and that protons with hyperfine couplingconstants of opposite sign show CIDNP effects of oppositesign.36 The Radical Pair Theory emerging from the work ofthe Chicago and Leiden groups is now generally acceptedand can explain the vast majority of all nuclear spin polar-ization effects.

In more than 20 additional publications Closs and col-laborators elucidated additional facets of the theory of CIDNPor dealt with applications to new problems. They made ma-


jor contributions to understanding biradicals: The magneticfield dependence of CIDNP effects of biradicals yields theiraverage singlet-triplet splitting,37 as well as their lifetimes(see Figure 3).38 Closs and colleagues also established re-laxation39 and cross relaxation phenomena.40 Cross relax-ation transfers nuclear spin polarization to nuclei, whichare not coupled to an unpaired electron spin. The exist-ence of cross relaxation complicates the interpretation ofCIDNP results, particularly in experiments designed to de-rive electronic structures of free radicals or radical ionsfrom their polarization pattern.

FIGURE 3 Normalized intensity versus magnetic field of aldehyde protonsignals obtained by photolysis of cycloalkanones and a bicyclic ketone (BC).All signals are in emission.37,38

Closs also pioneered flash photolysis with CIDNP detec-tion, combining the advantages of TRLS with the structural


information content unique to CIDNP.41-43 “The basis forthe success of the time-resolved method is the fact thatgeminate processes are complete in a fraction of a micro-second, while combination of free ions and/or exchange. . . may take tens or hundreds of microseconds dependingon concentration.”42 This method was soon adopted in otherlaboratories for various applications. The Closs group ap-plied this technique to determine the spin density distribu-tion in radical ions of chlorophyll and derivatives. The CIDNPintensities 1 µs after a photoinduced electron transfer reac-tion revealed the signs and relative magnitudes of hyper-fine coupling constants43 in good agreement with ENDORspectroscopy results. Another application probed the kinet-ics of triplet states and biradicals; photolysis of deoxybenzoincleaves a C–C bond next to the carbonyl group. The result-ing radical pair can regenerate the reactant by geminaterecombination; free radicals escaping from the geminatecage may have secondary encounters, forming deoxybenzoinor bibenzyl. At the shortest delay then attainable (1 µs) thereactant showed the expected geminate polarization; as ex-pected, when the delay time was increased to 100 µs, thissignal grew, due to contributions from free radical combi-nation (see Figure 4).41

O O

•

•

hν

rec

Without any doubt Gerhard Closs’s contributions havemade the CIDNP method a well-defined, sophisticated, pow-erful, and reliable technique with a wide spectrum of im-portant applications. When he first came to Chicago, super-


FIGURE 4 Normalized line intensities of bibenzyl (A) and deoxybenzoin(B), observed during the photolysis of deoxybenzoin, as a function of delaytime T. The deoxybenzoin polarization in the presence of a (thiol) freeradical scavenger is designated C.41

In the final decade of his life Gerhard Closs becameinterested in several aspects of chemically induced dynamicelectron polarization (CIDEP), a phenomenon related toCIDNP and discovered several years earlier.44 Essentially allCIDEP effects observed in the 20 years following the discov-ery could be explained by the radical pair or the tripletmechanism or by a combination of both. In the mid-l980sseveral time-resolved CIDEP spectra with highly unusual fea-

vision of the departmental NMR instrument was one of thetasks assigned to him. Surely Gerhard Closs took this re-sponsibility seriously and made the most of the opportuni-ties this position offered.


tures were observed upon photolysis of ketones in micelles,45

which could not be explained by the existing theories. As anatural extension of his earlier work on biradicals and inthe context of his interest in the superexchange mecha-nism of intramolecular electron transfer (see below), GerhardCloss investigated related systems in collaboration with M.Forbes and J. Norris. They recognized the unusual effectsas manifestations of electron spin-spin interactions; the re-sulting effects are rapidly lost in solution but they “remainobservable because of limited diffusion in micelles.”46

McLauchlan and coworkers derived a similar explanationindependently.47 The concept of spin-correlated radical pairsis now generally accepted.

Closs’s time-resolved electron spin resonance studiesculminated in several elegant studies of electron spin polar-ized polymethylene biradicals in solution, in which through-bond interactions were illuminated.48 Simulating the shapeand time dependence of CIDEP spectra from acyl-alkyl andalkyl-alkyl polymethylene biradicals by perturbation theoryyielded the electron spin-spin interaction, J, and the end-to-end contact rate, which is inversely related to the life-time. It is remarkable that Gerhard Closs was still breakingnew ground at the onset of his seventh decade.

In 1979 Gerhard Closs accepted the position of sectionhead in the Chemistry Division at Argonne National Labo-ratory. He kept this position for only three years, but thisappointment significantly influenced the direction of hisresearch in the final decade of his life. He took an interestin the electron transfer work of John Miller, who had evi-dence for the “inverted Marcus region.” More than threedecades earlier Marcus formulated the rate of an electrontransfer reaction as a function of two parameters, its driv-ing force (i.e., the free energy), ∆G0, of the reaction and a


“solvent reorganization energy,” λs, required to accommo-date the changing charge distribution.49

kET = A′ exp–[(∆G0 + λs)2 (4λskBT)–1]

The most striking result emerging from this work wasthe prediction that electron transfer rates increase with in-creasing driving force to a maximum at λs = ∆G0, but thenunexpectedly (and perhaps counterintuitively) increasingthe driving force further would decrease the reaction rate.The essential predictions of this theory were reproduced bynumerous theoretical approaches descending from the origi-nal idea of Marcus. It took almost 30 years before this pre-diction was confirmed by experiment.

Electron transfer reactions fall into three classes: chargeseparation, the photoinduced generation of radical ion pairsfrom a donor and an acceptor molecule; charge recombi-nation, the reverse process; and electron exchange betweena charged and a neutral entity. Rehm and Weller studiedthe charge separation in more than 60 organic donor-ac-ceptor pairs. The rate constants of fluorescence quenchingindicated a maximum rate of electron transfer, essentiallydiffusion limited, without evidence for an inverted region.Although at variance with the existing theories, the resultswere rationalized based on an ad hoc theory.50 Similar re-sults were observed in additional systems; all attempts toverify the predicted inverted free energy dependence ofelectron transfer rates met with failure.

Miller and coworkers had studied the (charge neutral)electron transfer from radiolytically generated radical an-ions to aromatic hydrocarbons in frozen solutions with freeenergy changes ranging from 0.01 < –∆G0 < 2.75 eV. Therates of electron transfer decreased at high exothermici-


ties,51 the first report of a reduction of electron transferrates with increasing driving force. However, because theentities were randomly distributed in rigid glasses, the re-sults were difficult to interpret and to understand.

In the collaboration that ensued and continued toGerhard Closs’s death donor and acceptor were linked by arigid steroid spacer in molecules of the type A-Sp-biphenyl.The electron transfer rates observed for the radiolyticallygenerated monoanions of these systems showed a strikingdeviation from the classical Brønsted relationship (see Fig-ure 5),52 confirming the predictions of Marcus. Less thanfive months after Gerhard Closs’s death, R. A. Marcus wasawarded the 1992 Nobel Prize in chemistry. In the announce-ment the Swedish Academy disclosed that the Nobel com-mittee had long recognized the significance of Marcus’stheory but had waited for experimental verification beforeawarding the prize.

These findings stimulated major research efforts aroundthe world. Within a year the Marcus inverted region forcharge recombination was reported and elaborated; a chargeseparation reaction was also found to exhibit the full rangeof Marcus behavior. The Chicago-Argonne team continuedto produce a host of significant results. They establishedthe nonlinear dependence of electron transfer rates upon

A

•–


solvent polarity, probed the distance dependence usingdecalin and cyclohexane spacers, and established the tem-perature independence of electron transfer rates in severalsystems. Having studied both “hole” and electron transferrates, Closs and Miller reasoned that triplet energy transfermight be considered as the sum of the two processes. Thisconsideration spawned an illuminating, unprecedented com-parison of the rates of hole, electron, and triplet energytransfer.53 Further collaboration was cut short by his un-timely death.

FIGURE 5 Intramolecular rate constants as a function of free energy changein 2-methyloxacyclopentane solution at 296 K. The electron transfer occursfrom biphenyl anions to the eight different acceptor moieties (shown adja-cent to the data points), in eight bifunctional molecules of the generalstructure shown in the center.52


One significant aspect of the electron transfer work sug-gested that donor and acceptor are coupled by the interac-tions with the orbitals of the intervening molecular frag-ments, a mechanism referred to as superexchange. Thiswas one of the reasons that caused Gerhard Closs to recon-sider the interaction of two unpaired electron spins inbiradicals through the intervening molecular fragment, lead-ing to the concept of spin-correlated radical pairs (see above).

Gerhard Closs made significant contributions in threefields of chemistry. He was an early leader in the field ofcarbene chemistry, he pioneered applications of magneticresonance to characterize reaction intermediates, and heelucidated intricate facets of electron transfer chemistry.He will be remembered for the depth and breadth of hisunderstanding and for his rigorous, all-encompassing ap-proach to research. Once he became interested in a prob-lem he would first master the theoretical background, ordevelop it himself, as was the case with CIDNP. Then hewould identify key features that needed to be verified anddesign ingenious and decisive experiments to probe thetheory. Finally, he would synthesize the selected target mol-ecules and conduct the physical experiments.

IN WRITING THIS MEMOIR I have benefited from conversations withseveral of Gerhard’s students and colleagues, particularly in thedifficult task of choosing the “Selected Bibliography” from his manysuperb publications: Robert A. Moss, who was a student in the carbeneyears (cf., 1964); John Miller, who inspired Gerhard’s interest inelectron transfer and collaborated with him for his final 10 years(cf., 1983, 1984, 1986, 1989, 1990); Jim Norris, an associate andfriend at the University of Chicago, who made available a tribute toGerhard, written for the University of Chicago Chronicle;54 and PiotrPiotrowiak, one of his last students, who bridged the electron trans-fer and biradical projects (cf., 1989, 1992).


NOTES

1. G. Wittig and L. Pohmer. Intermediäre Bildung vonDehydrobenzol (Cyclohexa-dienin). Angew. Chem. 67(1955):348.

2. G. Wittig L. Pohmer. Über das Intermediäre Auftreten vonDehydrobenzol Chem. Ber. 89(1956):1334-51.

3. G. Wittig, F. Mindermann, and G. L. Closs. Über Ringerweiterungund Ringverengerung auf der Basis von Ylidisomerisationen. LiebigsAnn. Chem. 594(1955):89-118.

4. R. B. Woodward, W. A. Ayer, J. M. Beaton, F. Bickelhaupt, R.Bonnett, P. Buchschacher, G. L. Closs, H. Dutler, J. Hannah, F. P.Hauck, S. Itô, A. Langemann, E. Le Goff, W. Leimgruber, W. Lwowski,J. Sauer, Z. Valenta, and H. Volz. The total synthesis of chlorophyll.J. Am. Chem. Soc. 82(1960):3800-3802

5. G. L. Closs and N. W. Gable. Studies toward the isolation ofthe active constituents of Panaeolous venenosus. Mycologia 11(1959):211-16.

6. J. U. Nef. J. Liebigs Ann. Chem. 270(1892):267; 280(1894):291;Über das zweiwerthige Kohlenstoffatom. 287(1895):265-59;298(1897):202.

7. A. Geuther. Über die Hydrolyse des Chloroforms in BasischemMedium. Liebigs Ann. Chem. 123(1862):121.

8. J. Hine. Carbon dichloride as an intermediate in basic hy-drolysis of chloroform. A mechanism for substitution reactions at asaturated carbon atom. J. Am. Chem. Soc. 72(1950):2438-45.

9. W. von E. Doering and A. K. Hoffmann. The addition ofdichlorocarbene to olefins. J. Am. Chem. Soc. 76(1954):2162-65.

10. G. L. Closs and L. E. Closs. Syntheses of chlorocyclopropanesfrom methylene chloride and olefins. J. Am. Chem. Soc. 81(1959):4996-97.

11. G. L. Closs and L. E. Closs. Addition of chlorocarbene tobenzene. Tetrahedron Lett. 10(1960):38-40.

12. G. L. Closs and L. E. Closs. Carbenes from alkyl halides andorganolithium compounds. III. Syntheses of alkyltropones from phenols.J. Am. Chem. Soc. 83(1961):599-602.

13. G. L. Closs and G. M. Schwartz. Ring-expansions of pyrroleand indole. J. Org. Chem. 26(1961):2609.

14. G. L. Closs, R. A. Moss, and J. J. Coyle. Steric course of somecarbenoid additions to olefins. J. Am. Chem. Soc. 84(1962):4985-86.


15. G. L. Closs and L. E. Closs. A novel synthesis of cyclopropenes.J. Am. Chem. Soc. 83(1961):1003-1004. Alkenylcarbenes as precur-sors of cyclopropenes. J. Am. Chem. Soc. 83(1961):2015-16.

16. R. G. W. Norrish and G. Porter. Chemical reactions producedby very high light intensities. Nature 164(1949):658.

17. G. Herzberg. The spectra and structures of free methyl andfree methylene. Proc. R. Soc. 262A(1961):291.

18. E. Zavoiskii. Paramagnetic relaxation of liquid solutions forperpendicular fields. J. Phys. U. S. S. R. 9(1945):211-16; Spin-mag-netic resonance in paramagnetic substances. J. Phys. U. S. S. R.9(1945):245; Paramagnetic absorption in solutions in parallel mag-netic fields. Zh. Eksp. Teor. Fiz. 15(1945):253-57. Paramagnetic ab-sorption in some salts in perpendicular magnetic fields. J. Phys. U.S. S. R. 10(1946):170-73.

19. R. W. Brandon, G. L. Closs, and C. A., Hutchison, Jr. Para-magnetic resonance in oriented ground-state triplet molecules. J.Chem. Phys. 37(1962):1878-79.

20. R. W. Murray, A. M. Trozzolo, E. Wasserman, and W. A. Yager.E. P. R. of diphenylmethylene, a ground-state triplet. J. Am. Chem.Soc. 84(1962):3213-14.

21. D. C. Doetschman and C. A., Hutchison, Jr. Paramagneticresonance and electron nuclear double resonance studies of thechemical reactions of diphenyldiazomethane and of diphenylmethylenein single 1,1-diphenylethylene crystals. J. Chem. Phys. 56(1972):3964-82.

22. G. L. Closs, C. A. Hutchison, Jr., and B. E. Kohler. Opticalabsorption spectra of substituted methylenes oriented in single crystals.J. Chem. Phys. 44(1966):413-14.

23. W. A. Gibbons and A. M. Trozzolo. Spectroscopy and photoly-sis of a ground-state molecule, diphenylmethylene. J. Am. Chem. Soc.88(1966):172-73.

24. I. Moritani, S.-I. Murahashi, M. Nishino, K. Kimura, and H.Tsubomura. Electronic spectra of the products formed by the pho-tolysis of diazo compound at 77 K, possibly identified to carbenes.Tetrahedron Lett. 4(1966):373-78.

25. G. L. Closs and B. E. Rabinow. Kinetic studies on diarylcarbenes.J. Am. Chem. Soc. 98(1976):8190-98.

26. I. Moritani, S.-I. Murahashi, H. Ashitaka, K. Kimura, and H.Tsubomura. Flash photolysis of 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptadiene. J. Am. Chem. Soc. 90(1968):5918-19.


27. R. B. Woodward and R. Hoffmann. The Conservation of OrbitalSymmetry. Weinheim: Verlag Chemie, 1971.

28. G. L. Closs and P. E. Pfeffer. The steric course of the thermalrearrangements of methylbicyclobutanes. J. Am. Chem. Soc. 90(1968):2452.

29. J. Bargon, H. Fischer, and U. Johnsen. Kernresonanz-Emissionslinien während rascher Radikalreaktionen. I.Aufnahmeverfahren und Beispiele. Z. Naturforsch. A. 22(1967):1551-55.

30. J. Bargon and H. Fischer. Kernresonanz-Emissionslinien währendrascher Radikalreaktionen. II. Chemisch induzierte dynamischeKernpolarization. Z. Naturforsch. A 22(1967):1556-60.

31. G. L. Closs and L. E. Closs. Induced dynamic nuclear spinpolarization in reactions of photochemically and thermally gener-ated triplet diphenylmethylene. J. Am. Chem. Soc. 91(1969):4549-50.

32. G. L. Closs and L. E. Closs. Induced dynamic nuclear spinpolarization in photoreductions of benzophenone by toluene andethylbenzene. J. Am. Chem. Soc. 91(1969):4550-52.

33. G. L. Closs. A mechanism explaining nuclear spin polariza-tions in radical combination reactions. J. Am. Chem. Soc. 91(1969):4552-54.

34. G. L. Closs and A. D. Trifunac. Chemically induced nuclearspin polarization as a tool for determination of spin multiplicitiesof radical-pair precursors. J. Am. Chem. Soc. 91(1969):4554-55.

35. G. L. Closs, C. E. Doubleday, and D. R. Paulson. Theory ofchemically induced nuclear spin polarization. IV. Spectra of radicalcoupling products derived from photoexcited ketones and aldehydes.J. Am. Chem. Soc. 92(1970):2185-86.

36. R. Kaptein. Chemically induced dynamic nuclear polarizationin five alkyl radicals. Chem. Phys. Lett. 2(1968):261.

37. G. L. Closs and C. E. Doubleday. Determination of the aver-age singlet-triplet splitting in biradicals by measurement of the magneticfield dependence of CIDNP. J. Am. Chem. Soc. 95(1973):2735-36.

38. G. L. Closs. Low-field effects and CIDNP of biradical reac-tions. In Chemically Induced Magnetic Polarization: Theory, Technique,and Applications, vol. 34, NATO Advanced Study Institutes Series,eds. L. T. Muus, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen,pp. 225-56. Dordrecht, Holland: D. Reidel, 1977.

39. G. L. Closs and M. S. Czeropski. Observation of a CIDNPpumped nuclear Overhauser effect: A caveat for the interpretationof CIDNP spectra. Chem. Phys. Lett. 45(1977):115-16.


40. G. L. Closs and M. S. Czeropski. Amendment of the CIDNPphase rules. Radical pairs leading to triplet states. J. Am. Chem. Soc.99(1977):6127-28.

41. G. L. Closs and R. J. Miller. Laser flash photolysis with NMRdetection. Microsecond time-resolved CIDNP: Separation of gemi-nate and random-phase processes. J. Am. Chem. Soc. 101(1979):1639-41.

42. G. L. Closs and E. V. Sitzmann. Measurements of degenerateradical ion-neutral molecule electron exchange by microsecond time-resolved CIDNP. Determination of relative hyperfine coupling con-stants of radical cations of chlorophylls and derivatives. J. Am. Chem.Soc. 103(1981):3217-19.

43. G. L. Closs and R. J. Miller. Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection inlaser flash photolysis experiments. Rev. Sci. Instrum. 52(1981):1876-85.

44. R. W. Fessenden and R. H. Schuler. Electron spin resonancestudies of alkyl radicals. J. Chem. Phys. 39(1963):2147-95.

45. Y. Sakaguchi, H. Hayashi, H. Murai, and Y. J. I’Haya, CIDEPstudy of the photochemical reactions of carbonyl compounds show-ing the external magnetic field effect in a micelle. Chem. Phys. Lett.110(1984):275-79; Y. Sakaguchi, H. Hayashi, H. Murai, Y. J. I’Haya,and K. Mochida. CIDEP study of the formation of cyclohexadienyl-type radicals in the hydrogen abstraction of triplet xanthone. Chem.Phys. Lett. 120(1985):401-405.

46. G. L. Closs, M. D. E. Forbes, and J. R. Norris, Jr. Spin-polar-ized electron paramagnetic resonance spectra of radical pairs inmicelles. Observation of electron spin-spin interactions. J. Phys. Chem.91(1987):3592-99.

47. C. D. Buckley, D. A. Hunter, P. J. Hore, and K. A. McLauchlan.Electron spin resonance of spin-correlated radical pairs. Chem. Phys.Lett. 135(1987):307-12.

48. G. L. Closs, M. D. E. Forbes, and P. Piotrowiak. Spin andreaction dynamics in flexible polymethylene biradicals as studied byEPR, NMR, and optical spectroscopy and magnetic field effects.Measurements and mechanisms of scalar electron spin-spin cou-pling. J. Am. Chem. Soc. 114(1992):3285-94.

49. R. A. Marcus. The theory of oxidation-reduction reactions in-volving electron transfer. I. J. Chem. Phys. 24(1956):966-78; The theory


of oxidation-reduction reactions involving electron transfer. II. Ap-plications to data on the rates of isotopic exchange reactions. J.Chem. Phys. 26(1957):867-71. The theory of oxidation-reduction re-actions involving electron transfer. III. Applications to data on therates of organic redox reactions. J. Chem. Phys. 26(1957):872-77.Theory of electrochemical and chemical electron-transfer processes.Can. J. Chem. 37(1959):155-63.

50. D. Rehm and A. Weller. Kinetics of fluorescence quenchingby electron and h-atom transfer. Israel J. Chem. 8(1970):259-71.

51. J. R. Miller, J. V. Beitz, and R. K. Huddleston. Effect of freeenergy on rates of electron transfer between molecules. J. Am. Chem.Soc. 106(1984):5057-68.

52. G. L. Closs and J. R. Miller. Intramolecular long-distance elec-tron transfer in organic molecules. Science 240(1988):440-47.

53. G. L. Closs, M. D. Johnson, J. R. Miller, and P. Piotrowiak. Aconnection between intramolecular long-range electron, hole, andtriplet energy transfers. J. Am. Chem. Soc. 111(1989):3751-53.

54. J. M. Norris. Gerhard Closs, 1928-1992. University of ChicagoChronicle 12(11), 1993.


S E L E C T E D B I B L I O G R A P H Y

1960

With L. E. Closs. Carbenes from alkyl halides and organolithiumcompounds. I. Synthesis of chlorocyclopropanes. J. Am. Chem.Soc. 82:5723-28.

1961

With L. E. Closs. Alkenylcarbenes as precursors of cyclopropenes. J.Am. Chem. Soc. 83:2015-16.

1963

With L. E. Closs. Carbon orbital hybridizations and acidity of thebicyclobutane system. J. Am. Chem. Soc. 82:2022-23.

1964

With R. A. Moss. Carbenoid formation of arylcyclopropanes fromolefins, benzal bromides, and organolithium compounds and fromphotolysis of aryldiazomethanes. J. Am. Chem. Soc. 86:4042-53.

1965

With R. L. Brandon, C. E. Davoust, C. A. Hutchison, Jr., B. E. Kohler,and R. Silbey. Electron paramagnetic resonance spectra of theground-state triplet diphenylmethylene and fluorenylidene mol-ecules in single crystals. J. Chem. Phys. 43:2006-16.

1969

With L. E. Closs. Induced dynamic nuclear spin polarization in re-actions of photochemically and thermally generated tripletdiphenylmethylene. J. Am. Chem. Soc. 91:4549-50.

With L. E. Closs. Induced dynamic nuclear spin polarization in pho-toreductions of benzophenone by toluene and ethylbenzene. J.Am. Chem. Soc. 91:4550-52.

A mechanism explaining nuclear spin polarizations in radical com-bination reactions. J. Am. Chem. Soc. 91:4552-54.

With A. D. Trifunac. Chemically induced nuclear spin polarizationas a tool for determination of spin multiplicities of radical-pairprecursors. J. Am. Chem. Soc. 91:4554-55.


1972

With C. E. Doubleday. Chemically induced dynamic nuclear spinpolarization derived from biradicals generated by photochemicalcleavage of cyclic ketones, and the observation of a solvent effecton signal intensities. J. Am. Chem. Soc. 94:9248-49.

1973

With C. E. Doubleday. Determination of the average singlet-tripletsplitting in biradicals by measurement of the magnetic field de-pendence of CIDNP. J. Am. Chem. Soc. 95:2735-36.

1975

With S. G. Boxer. Nuclear magnetic resonance of photoexcited trip-let states. I. The measurement of the rate of degenerate singlet-triplet exchange for anthracene in solution. J. Am. Chem. Soc.97:3268-70.

1976

With B. E. Rabinow. Kinetic studies on diarylcarbenes. J. Am. Chem.Soc. 98:8190-98.

1979

With S. L. Buchwalter. Electron spin resonance and CIDNP studieson 1,3-cyclopentadiyls. A localized 1,3 carbon biradical systemwith a triplet ground state. Tunneling in carbon-carbon bondformation. J. Am. Chem. Soc. 101:4688-94.

1981

With E. V. Sitzmann. Measurements of degenerate radical ion-neu-tral molecule electron exchange by microsecond time-resolvedCIDNP. Determination of relative hyperfine coupling constantsof radical cations of chlorophylls and derivatives. J. Am. Chem.Soc. 103:3217-19.

With R. J. Miller. Laser flash photolysis with NMR detection.Submicrosecond time-resolved CIDNP: Kinetics of triplet statesand biradicals. J. Am. Chem. Soc. 103:3586-88.

1983

With L. T. Calcaterra and J. R. Miller. Fast intramolecular electron


transfer in radical ions over long distances across rigid saturatedhydrocarbon spacers. J. Am. Chem. Soc. 105:670-71.

1984

With J. R. Miller and L. T. Calcaterra. Intramolecular long-distanceelectron transfer in radical anions. The effects of free energyand solvent on the reaction rates. J. Am. Chem. Soc. 106:3047-49.

1986

With L. T. Calcaterra, N. J. Green, K. W. Penfield, and J. R. Miller.Distance, stereoelectronic effects, and the Marcus inverted re-gion in intramolecular electron transfer in organic radical an-ions. J. Phys. Chem. 90:3673-83.

1988

With J. R. Miller. Intramolecular long-distance electron transfer inorganic molecules. Science 240:440-47.

1989

With M. D. Johnson, J. R. Miller, and N. S. Green. Distance depen-dence of intramolecular hole and electron transfer in organicradical ions. J. Phys. Chem. 93:1173-76.

With M. D. Johnson, J. R. Miller, and P. Piotrowiak. A connectionbetween intramolecular long-range electron, hole, and triplet energytransfers. J. Am. Chem. Soc. 111:3751-53.

With N. Liang and J. R. Miller. Correlating temperature depen-dence to free energy dependence of intramolecular long-rangeelectron transfers. J. Am. Chem. Soc. 111:8740-41.

1990

With N. Liang and J. R. Miller. Temperature-independent long-range electron transfer reactions in the Marcus inverted region.J. Am. Chem. Soc. 112:5353-54.

1992

With M. D. E. Forbes and P. Piotrowiak. Spin and reaction dynam-ics in flexible polymethylene biradicals as studied by EPR, NMR,and optical spectroscopy and magnetic field effects. Measurementsand mechanisms of scalar electron spin-spin coupling. J. Am. Chem.Soc. 114:3285-94.

GERHARD LUDWIG CLOSS 1928–1992 · three other fields of chemistry, one area for each decade of...

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